Digital telephone system noise reduction circuit

Abstract

A telephone system uses above voice-band ASK control and supervisory signals which are appended to digitally encoded voice by forcing the least significant bits to a selected level for transmission to the receiving end. At the receiving end, the ASK is regenerated by a circuit including special filter and a pulse generator. To eliminate audible noise resulting from the filter action, the ASK signal or the pulses from the pulse generator is shifted in phase by about 90° before application to a filter. The result is significantly reduced noise.

What is claimed is: 
     
       1. In a telephone system including means at a first location for generating amplitude shift key (ASK) control or supervisory signals at a frequency significantly above voice band and voice signals for transmission to a destination connected to said first location by voice quality telephone lines, means at a first location end of the telephone lines for digitally encoding the voice signals in a plurality of digital words and means for recognizing the presence of the ASK signals for setting a least significant bit (LSB( of each said digital word at one preselected state when an ASK signal is present and a second state when no ASK signal is present, and means at a destination end of the telephone lines for reconstructing the voice signals and for regenerating the ASK signal in response to the LSB states, and wherein said means for reconstructing includes filter circuit means responsive to the presence of the ASK signal to produce an output signal, the improvement comprising circuit means for receiving the ASK signal and for shifting the phase of said ASK signal before said signal is applied to said filter circuit means, and   a second circuit means for coupling said phase shifted signal to said filter circuit means to thereby reduce the amplitude of noise produced in said system.   
     
     
       2. A system according to claim 1 wherein said ASK signal is shifted in phase by 90°. 
     
     
       3. A method of reducing noise in a telephone system including means at a first location for generating amplitude shift key (ASK) control or supervisory signals at a frequency significantly above voice band and voice signals for transmission to a destination connected to said first location by voice quality telephone lines, means at a first location end of the telephone lines for digitally encoding the voice signals in a plurality of digital words and means for recognizing the presence of the ASK signals for setting a least significant bit (LSB) of each said digital word at one preselected state when an ASK signal is present and a second state when no ASK signal is present, and means at a destination end of the telephone lines for reconstructing the voice signals and for regenerating the ASK signal as a sequence of pulses in response to the LSB states, and wherein said means for reconstructing includes filter circuit means responsive to the presence of the ASK signal pulses to produce an output signal, the method comprising the step of shifting the phase of the pulses comprising the ASK signal by about 90°, and applying the phase shifted pulses to the filter circuit means to reduce the amplitude of noise produced.